1. Field of the Invention
This invention relates to an exposure parameter obtaining method in charged beam exposure using electron beams or the like, an exposure parameter evaluating method, a semiconductor device manufacturing method, a charged beam exposure apparatus and to a method of the same.
2. Description of the Related Art
With the recent higher-density packaging of large-scale semiconductor integrated circuits, charged beam exposure apparatuses using a charged particle beam, such as an electron beam, have been put to practical use. For instance, an electron beam exposure apparatus uses a variably shaped beam (VSB) obtained by variably shaping the cross section of an electron beam generated at an electron beam source. The variably shaped beam is directed onto a waver surface and deflected for scanning on the surface of the wafer according to a pattern data, thereby drawing a desired pattern. That is, this type of electron beam exposure apparatus has a pattern generation function of forming a pattern, hardware, on the waver from the pattern data, software.
Since the electron beam exposure apparatus draws a pattern by connecting exposure shots on the wafer with a variably shaped beam whose cross section has been shaped into, for example, a rectangle or a triangle, the smaller the pattern size to be drawn, the electron beam generally has to be the finer. As a result, the number of exposure shots per unit area increases and therefore the throughput tends to decrease.
On the other hand, in the case of manufacturing a semiconductor device, such as memory, which needs ultrafine pattern exposure, although patterns to be exposed are fine, most of them are often composed of repetitions of basic patterns. Therefore, if a basic pattern or a character pattern, a unit of repetitive pattern, is generated by a single shot, such an ultra-fine pattern could be exposed with a relatively high throughput, even when the basic pattern is rather complex.
Therefore, instead of an exposure method using a variably shaped beam, an electron beam exposure apparatus is being put to practical use which is performed by employing a drawing method using basic pattern projection techniques. In this method, such basic pattern is called a character. In the exposure apparatus, an electron beam is projected onto the wafer through a selected aperture having a character shape of a beam shaping mask. The mask has a plurality of basic patterns, or a plurality of characters, and is called as a character projection pattern (hereinafter, referred to as a CP pattern) mask, thereby producing an electron beam having a basic pattern section of a desired shape with a single shot.
As described above, an electron beam exposure apparatus is being put to practical use which has employed a drawing method using character projection techniques for connecting basic pattern shots by exposing the patterns repeatedly, and thereby achieving a practically high throughput.
The total wafer drawing time of the electron beam exposure apparatus is expressed roughly by the product of an exposure time required to expose a single character and the number of shots. Therefore, when the resist sensitivity is raised and the beam current density is increased to shorten the shooting time and the beam size is made larger to decrease the number of shots, the drawing time is shortened. Since the character projection technique (hereinafter, referred to as the CP technique) for transferring basic patterns repeatedly decreases the number of shots, it has a higher throughput than that of the variably shaped beam technique (referred to as the VSB technique). When several hundreds CP patterns are formed on a single mask, the frequency of mask replacement decreases, which improves the throughput remarkably. The exposure method using the conventional CP technique, however, has the following problem.
In the electron beam exposure apparatus using the CP technique, before exposure a wafer by an electron beam, the electron beam passed through each CP pattern aperture selected from a single CP pattern mask has to be so adjusted that the beam reaches a specific position with respect to a reference pattern previously defined on the wafer. If there is any one of a shift in position, rotation, blurring, and a fluctuation in the magnification in the CP pattern drawn on the wafer by the electron beam, accurate pattern exposure is impossible for the CP pattern. Accordingly, it is very important to correct or offset the position on the wafer of the electron beam passed through each aperture corresponding to each CP pattern.
In a conventional method of correcting the position of the CP pattern, for example, an area including a microscopic mark made of a heavy metal put on a wafer is scanned with an electron beam generated at an exposure apparatus. Then, secondary electrons generated at the microscopic mark at that time are detected to obtain an electron signal image of the electron beam shape or CP pattern. Then, pattern matching between the obtained image and a reference pattern is effected. From the difference in position between them, the amount of correction of the beam exposure position, or exposure parameter, is determined.
In a conventional method, it is necessary to generate an electron signal image of each CP pattern by scanning the electron beam repeatedly over each pattern selected from the mask and used for exposure. As a result, it takes a lot of time to adjust the position of each CP pattern and accordingly it is virtually difficult to obtain the exposure parameter for each of several hundreds CP patterns.